command: implement A-B loops

Probably needs to be polished a bit more. Also, might require a key
binding that can set/clear the loop points in a more intuitive way.

For now, something like this can be put into input.conf to use it:

ctrl+y set ab-loop-a ${time-pos}    # set A
ctrl+x set ab-loop-b ${time-pos}    # set B
ctrl+c set ab-loop-a no             # clear (mostly)

Fixes #1241.

DOCS/man/input.rst

@@ -778,6 +778,9 @@ Property list
                 "title"             MPV_FORMAT_STRING
                 "default"           MPV_FORMAT_FLAG
 
+``ab-loop-a``, ``ab-loop-b`` (TW)
+    Set/get A-B loop points. See corresponding options.
+
 ``angle`` (RW)
     Current DVD angle.
 

DOCS/man/options.rst

@@ -229,6 +229,12 @@ Playback Control
     between the two option is that this option performs a seek on loop, instead
     of reloading the file.
 
+``--ab-loop-a=<time>``, ``--ab-loop-b=<time>``
+    Set loop points. If playback passes the ``b`` timestamp, it will seek to
+    the ``a`` timestamp. Seeking past the ``b`` point doesn't loop (this is
+    intentional). The loop-points can be adjusted at runtime with the
+    corresponding properties.
+
 ``--ordered-chapters``, ``--no-ordered-chapters``
     Enabled by default.
     Disable support for Matroska ordered chapters. mpv will not load or